Voltage source inverters are increasingly utilized in many applications, such as, for example, motor drive systems for electric vehicles or hybrid-electric vehicles within automotive applications. In these applications, a voltage source inverter (VSI) may supply an alternating current (AC) dependant device, such as, for example, an AC motor.
Thermal management issues concerning power switches within the voltage source inverter (VSI) may arise when the VSI operates at low or zero output frequency, that is, the VSI is applying zero or low voltage across the phases while receiving a constant direct current (DC) link voltage, referred to as input voltage. When operating at low or zero output frequency, the average voltage produced by the VSI is low in comparison to the DC link voltage, while the output current might be the switch rated current.
Thermal management issues arise at zero or low output frequency due to slowly changing AC current. That is, under these conditions, a power switch in the VSI might be required to carry maximum rated current continuously or for a very long period, causing constant high power dissipation inside the power switch. This condition represents a worst-case operating condition for a switch in the VSI. A conventional solution is to time-limit the VSI operation at zero output frequency and full output current. Unfortunately, this conventional solution is problematic for electric and hybrid vehicle operation because it limits the time during which maximum torque can be applied to the electric motor during vehicle start.
It would be desirable, therefore, to provide a system that would overcome these and other disadvantages.